Along with a marked increases in power generation to meet an increased demand for power supply in the recent years, problems of environmental pollution have become serious in the world.
In fact, atomic power generation was anticipated as a means of generating power capable of replacing steam-power generation and has been in operation in some places of the world. However, there were occurrences represented by Chernobyl nuclear power plant disaster, where the systems broke down to cause radioactive contamination. Because of this, further development of the system of atomic power generation is feared. There are some countries that effected regulations prohibiting establishment of new atomic power plants.
Now, in the case of the generation of steam power, the amount of a fossil fuel represented by coal or petroleum to be consumed for power generation in order meet the social demand for increased power supply has been continuously increased and along with this, the amount of carbon dioxide gas exhausted from the steam-power generation plants has been continuously increased resulting an increase in the carbon dioxide content of gases in the air to cause a greenhouse effect. This results in providing a global-warming phenomenon. In fact, the annual average temperature of the earth has increased in the recent years. In view of this, International Energy Agency has proposed reducing the amount of carbon dioxide exhausted from the steam-power generation plant by as much as 20% of the current level by the year of 2005.
Against this background, the populations of the developing countries will continue to increase and a demand for power supply will also be increased. In addition to this, it is expected that the manner of living in the developed countries will be further modernized. With further developments in electronic instruments the amount of power consumption per person will be eventually increased.
In view of the above, the matter of power supply is now a subject to be internationally discussed in terms of the earth.
Public attention has now focused on and various studies have been made of generating power using a solar cell since it has various advantages: it is a clean power generation system which is free of problems relating to radioactive contamination, global-warming and environmental pollution; the sunlight to be used as its energy source reaches everywhere on the earth and there is not a problem for the energy source to be localized; and the power generation equipment can be simplified and a relatively high power generation efficiency can be attained.
Now, in order for the solar cell power generation system to supply power in a quantity to satisfy the social demand, the solar cell to be used should provide a sufficiently high photoelectric conversion efficiency, it should stably exhibit solar cell characteristics an d it should be mass-produced.
In order to provide the average family with adequate power, a solar cell capable of outputting a power of about 3 KW is necessary. In this case, the photoelectric conversion efficiency of the solar cell should be about 10%, the solar cell is required to have an area of about 30 m.sup.2 in order to provide said power. And in the case where it is intended to satisfy the demands for power supply for 100,000 families, the solar cell is required to have an area of 3,000,000 m.sup.2.
In view of this, public attention has been focused on an amorphous silicon solar cell which can be prepared by depositing a semiconductor film such as an amorphous silicon semiconductor film on a relatively inexpensive substrate such as glass or metal sheet wherein glow discharge is caused in a film-forming raw material gas such as silane gas. Such a solar cell can be mass-produced and can be provided at a lower cost in comparison with a solar cell prepared by using a single crystal silicon or the like. Various proposals have been made for the process of producing said amorphous silicon solar cell.
In the case of the power generation system using a solar cell, usually a plurality of unit modules are connected in series or in a row to form a unit from which a desired current or voltage can be obtained. For each of said plurality of modules, it is required that neither disconnection nor shunt circuit occur. It is further required that each of said plurality of modules stably outputs an even current or voltage. In order to satisfy these requirements, each unit module is necessarily prepared such that its most important constituent semiconductor layer stably exhibits the uniform characteristics required therefor. Further, to make it easy to design the module and also to simplify the process for assembling a plurality of unit modules in to a unit, it is essential to provide a large area semiconductor film having an uniformity not only in thickness but also in quality and capable of exhibiting uniform semiconductor characteristics. These features enable the mass-production of a solar cell and reduce the production cost. In the solar cell, the constituent semiconductor layers, which are the basic constituent elements thereof, are conjugated to form a semiconductor junction such as pn junction or pin junction. These semiconductor junctions can be attained by stacking different semiconductor layers having a different conduction type one from another, or by ion-implanting or thermally diffusing a dopant of a different conduction type into one of the constituent semiconductor layers of the same conduction type.
This situation will be further described in the case of the foregoing amorphous silicon solar cell. It is known that glow discharge is caused in a gas mixture composed of a film-forming raw material gas such as silane gas and a raw material gas capable of supplying a dopant element such as phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6) forming semiconductor films having a desired conduction type and a desired semiconductor junction can be easily attained by sequentially stacking these semiconductor films on a desired substrate. For instance, in order to prepare an amorphous silicon series solar cell, there has been proposed a method wherein a plurality of independent film-forming chambers for forming the respective semiconductor films therein are provided and each of the semiconductor films is formed in the corresponding film-forming chamber.
The specification of U.S. Pat. No. 4,400,409 discloses a continuous plasma CVD apparatus wherein the so-called roll-to-roll system is employed. The specification describes that said apparatus makes it possible to continuously form an element having a semiconductor junction by providing a plurality of glow discharge regions through each of which regions a lengthy flexible substrate having a desired width is to be moved. A semiconductor film of a desired conduction type is formed on said substrate in each of said plurality of glow discharge regions while continuously moving said substrate in the longitudinal direction. The specification describes that a gas gate is provided between the adjacent glow discharge regions in order to prevent a raw material gas to be used in one glow discharge region from entering into the other glow discharge region. In more detail, said plurality of glow discharge regions are isolated one from the other by an isolation passage way provided with means for forming a cleaning gas stream of Ar, H.sub.2, etc. It can be said that this roll-to-roll system will be suitable for the mass-production of a semiconductor device. However, since each of the constituent semiconductor layers is formed by the plasma CVD method using a RF energy, there is a limit for continuously forming those constituent semiconductor layers at a high deposition rate while maintaining the characteristics desired for each of those constituent semiconductor layers. That is, even in the case of forming a thin semiconductor layer of, for example, about 5000 .ANG., it is necessary to always produce prescribed plasma and sustain the plasma in a uniform state over a large area. However, there are many correlated film-forming parameters which are difficult to generalize and highly skilled techniques are required to do so. In addition to this, there are still other unresolved problems that the decomposition rate and the utilization efficiency of a film-forming raw material gas are not sufficient and thus the product becomes unavoidably costly.
Japanese Unexamined Patent Publication Sho. 61-288074 discloses a roll-to-roll film-forming apparatus comprising a reaction chamber containing a hood-like shaped curtaining portion of a flexible substrate web, said reaction chamber having a reaction space circumscribed by said hood-like shaped curtaining portions and said reaction chamber being provided with at least an activation chamber isolated from said reaction chamber. The film formation is carried out by introducing active species formed in said activation chamber and if necessary, other film-forming raw material gas into said reaction space and chemically reacting them with the action of a heat energy to form a deposited film on the inner surface of said hood-like shaped curtaining portion positioned in said reaction chamber. This apparatus is advantageous in that the apparatus can be relatively compact and the deposition rate of a film to be formed may be improved because of using an active species in comparison with the known plasma CVD apparatus. However, this film-forming apparatus utilizes chemical reaction to cause film formation with the aid of a heat energy. Therefore, when the deposition rate of a film to be formed is desired to be increased, it is necessary to increase not only the flow rate of an active species into the reaction space but also the quantity of a heat energy to be supplied. However, it is extremely difficult to do so since there is a limit not only for the manner of generating a large amount of the active species in the activation chamber and sufficiently introducing the active species into the reaction space at a high flow rate without leakage but also for uniformly supplying a large quantity of the heat energy into the reaction space.
In addition, since the means of providing the hood-like shaped curtaining portion comprises merely two transportation rollers for transporting the substrate web and the hood-like shaped curtaining portion is formed by the self-holding power of the substrate web, there are problems such that the inside volume of the space circumscribed by the substrate web is liable to change, resulting in a change in the film-forming conditions. The substrate temperature is liable to suffer from influence of a change in the pressure in the space, resulting in unevenness in the characteristics of a film formed, since the calorific capacity of the substrate web itself is relatively small. Thus, there is a serious disadvantage that it is required to adjust the substrate temperature at the position in contact with the substrate web.
In recent years, a plasma process using microwave has been spotlighted because microwave makes it possible to provide an energy density which is higher than that provided by RF in the prior art and it is suitable for effectively producing and maintaining plasma, since microwave is short in frequency band.
For instance, the specifications of U.S. Pat. Nos. 4,517,223 and 4,504,518 describe methods for forming deposited thin films on small area substrates in a microwave glow discharge plasma under a low pressure condition. These two patent specifications describe that because the methods are conducted under low pressure conditions, any of these methods makes it possible to obtain a high quality deposited film at a remarkably high deposition rate while eliminating not only polymerization of active species which gives negative effects to the characteristics of a film to be formed but also formation of powder such as polysilane in the plasma. However, neither of these two patent specifications mentions anything about uniform deposition of a film over a large area.
The specification of U.S. Pat. No. 4,729,341 discloses a low pressure microwave plasma CVD method and an apparatus suitable for practicing the same, wherein a photoconductive semiconductor thin film is deposited on a large area cylindrical substrate using a pair of radiative waveguide applicators in a high power process. However, the principles of large area film deposition are limited to cylindrical substrates for electrophotographic photoreceptors, and the teachings described therein are not directly transferable to planar substrates of large area. Further, the film-forming method is to be practiced in a batch system and the amount of film products obtained by one batch is limited. The specification does not teach anything about continuous film deposition on a large area planar substrate.
Now, there still remain various problems to be solved for large area film deposition because non-uniformity of a microwave energy is apt to occur in microwave plasma due to the wavelength of a microwave being short. For example, there is an attempt to use a slow microwave structure in order to provide uniformity of the microwave energy. However, there is an inherent problem in the slow microwave structure in that there is the very rapid fall off of microwave coupling into the plasma as a function of distance transverse to the microwave applicator. In order to solve this problem, a proposal has been made that the spacing of the slow microwave structure from a substrate to be processed is varied to thereby make the energy density in the vicinity of the substrate surface uniform.
For example, such a proposal can be found in the specification of U.S. Pat. No. 3,814,983 or the specification of U.S. Pat. No. 4,521,717. More particularly, the former patent specification discloses that it is necessary to incline the slow wave structure at a certain angle with respect to the substrate. However, inclination of the slow wave structure leads to an insufficient coupling of a microwave energy into the plasma. The latter patent specification discloses the use of two slow wave structures in an anti-parallel arrangement but in parallel to the substrate. More particularly, the latter patent specification discloses that it is desirable to set the two slow wave applicators at an angle to each other so that the planes normal to the medians of the applicators intersect at a straight line which extends parallel to the surfaces of the substrate to be processed and at right angles to the travel direction of the substrate; and that in order to avoid destructive interference between the two s low wave applicators, it is desirable to displace the two slow wave applicators from each other transversly of the travel direction of th e substrate by a distance equal to half of the space between the cross-bars of th e waveguide.
There have been several proposals made in order to provide plasma uniformity (that is, energy uniformity). Such proposals are found, for example, in Journal of Vacuum Science Technology, B-4 (January-February of 1986) PP. 295-298 and in the same document, (January-February of 1986) PP. 126-130. These reports describe a microwave reactor called a microwave plasma disc source (MPDS) and that the plasma is in the shape of a disc or tablet, with a diameter that is a function of microwave frequency. More particularly, the reports describe that the plasma disc source can be varied with the frequency of microwave. However, in the case of a microwave plasma disc source which is designed for operation at the normal microwave frequency of 2.45 GHz, the plasma confined diameter is about 10 cm at the most and the plasma volume is about 118 cm.sup.3 at the most. This is far from a large surface area. The reports also describe that in the case of a system designed for operation at the lower frequency of 915 MHz, the lower frequency source would provide a plasma diameter of approximately 40 cm with a plasma volume of 2000 cm.sup.3. The reports further describe that the microwave plasma disc source can be scaled up to discharge diameters in excess of 1 m by operating at still lower frequencies, for example, 40 MHz. However extreme expenses are required to establish such an apparatus which can perform this. More particularly in this respect, it will be possible to attain large area plasma by lowering the frequency of microwave employed. But the high power microwave power source with such frequency region is not generally available. If it should be accessible, it will be extremely costly. In addition, as for the frequency variable high power microwave power source, it is difficult to acquire such power source.
In order to effectively provide high density plasma using microwave, there have been ways proposed to establish the electron cyclotron resonance condition (namely, the ECR condition) by arranging electromagnets around the cavity resonator as found in Japanese Unexamined Patent Publications Sho. 55-141729 and Sho. 57-133636. At academic meetings, etc., there have been methods reported of forming various semiconductor thin films by utilizing high density plasma. Some microwave ECR plasma CVD apparatus capable of performing such methods have been commercialized.
However, it has been generally recognized in the technical field to which the invention pertains that it is technically difficult to form a deposited film uniformly over a large area substrate because of non-uniformity of plasma caused by the wavelength of microwave and also because of non-uniformity of magnetic field distribution due to the use of the magnets for the control of plasma. In the case where the microwave ECR plasma CVD apparatus is intended to be scaled up so that film deposition over a large area can be done, there are various problems to be solved: 1) electro-magnets to be used must also be scaled up; 2) means for preventing the apparatus from overheating must be provided; and 3) a special DC high power regulated supply must be provided.
Further, as for the deposited film obtained, it is usually inferior to the deposited film obtained by the known RF plasma CVD method with respect to the film property. In addition, in the case of forming a deposited film on a substrate by the microwave ECR plasma CVD method, there is a distinguishable difference with respect to the film deposition rate and the film property between the film formed in the space where the ECR condition is established and the film formed in the space where the ECR condition is not established, in the dispersed magnetic field space in other words. In view of this, the microwave ECR plasma CVD method is not suitable for the preparation of such a semiconductor device that is required to excel in quality and in uniformity with respect to the characteristics to be provided.
The foregoing U.S. Pat. Nos. 4,517,223 and 4,729,341 describe the necessity of maintaining very low pressures in order to provide high density plasmas. That is, they describe that the use of low pressures is necessary in order to obtain a high film deposition rate and/or a high gas utilization efficiency. However, the foregoing slow wave structure and electron cyclotron resonance method is not sufficient to maintain the relationships among high film deposition rate, high gas utilization efficiency, high power density and low pressure.
In view of the problems described above, there is an increased demand for eliminating the foregoing problems of the known microwave plasma CVD method and providing an improved microwave plasma CVD process which is free of such problems.
There is also another demand for providing a large area or lengthy thin semiconductor film excelling in quality and uniformity of characteristics which is desirably usable in not only solar cells but also in semiconductor devices such as TFTs, photoelectric conversion elements for contact image sensors, switching elements, image input line sensors, etc. at a reduced cost.